This invention relates to a method of producing chemical pulp in an impregnation stage and a cooking stage, using a hydraulic digester, especially a single-vessel hydraulic digester. The invention relates also to a digester system and to a steam injector.
Continuous digesters are widely used to produce chemical pulp. There are essentially two main types of continuous digesters: the hydraulic digester and the vapor-phase digester. A hydraulic digester is a pressure-resistant vessel which is completely filled with comminuted cellulosic fibrous material and liquid; any introduction or removal of liquid from the vessel affects the typically super-atmospheric pressure within the vessel. A vapor-phase digester is not completely filled with liquid but includes a section at the top containing super-atmospheric steam. Since this gas zone is compressible compared to the liquid zone below it, the pressure within a vapor-phase digester is typically determined by the pressure of the gas present at the top of the digester. The reaction of pulping chemicals with comminuted cellulosic fibrous material to produce a chemical pulp requires temperatures ranging between 140-180° C. Since at atmospheric conditions the aqueous chemicals used to treat the material would boil at such temperatures, commercial chemical pulping is typically performed in a pressure-resistant vessel under pressures of at least about 5 bars gauge.
One principal distinction between the method of operation of these two types of digesters is the way the contents of the digesters are heated to the desired 140-180° C. In the vapor-phase digester, the chips are typically heated by exposing the chips to steam. This steam heating is typically performed as the chips are introduced to the steam-filled zone at the top of the digester. In the hydraulic digester, the slurry of comminuted cellulosic fibrous material, typically wood chips, and cooking liquor is typically heated by means of heated liquid circulations, i e. one or more recirculation loops. Liquid is typically removed from the digester, for example, by using an annular screen assembly and pump, heated with steam by means of an indirect heat ex-changer, and re-introduced to the material in the vessel using a centrally located pipe. It has not been possible to add direct steam to the top of the hydraulic digester because the steam condensing into liquor would have caused hammering and in the worse it could have caused cracks to the digester shell. In some cases a steam line has been connected to the top of the hydraulic digester, but purpose of this steam has been to push the digester empty of chips and liquor before the shutdown, not to use it for heating during normal operation.
Furthermore, chips are introduced to the digesters using different mechanical devices. Wood chips, or other comminuted cellulosic fibrous material, are typically fed to the inlet of a continuous digester using a separate feed system. The feed system typically includes equipment for de-aerating, heating, pressurizing, and introducing cooking liquor to the chips before transferring a slurry of chips and liquor to the digester. In the case of the hydraulic digester, this slurry of chips and liquor is introduced in a downward-directed screw-type conveyor at the top of the digester, known in the art as a “top separator”.
The digester chip feed systems can be divided into two classes: Systems which have only atmospheric steaming to heat the chips and remove air from the chips, and systems which have both atmospheric and pressurized steaming. If there is only atmospheric steaming the temperature level at feed system is typically about 100° C. If there is also pressurized steaming, where the pressure is typically 0.7 to 1.5 bar higher than the atmospheric pressure, the temperature level is typically from 115 to 125° C. There is no additional heating between the feed system and the top of a single-vessel hydraulic digester and the temperature in the impregnation zone at the top is at the same level as in the feed system. Cooking temperature in the cooking zone is typically between 140° C. and 180° C. So there is a large temperature difference between the impregnation zone temperature at the top of the single-vessel hydraulic digester and the cooking zone. Due to the large temperature difference it can be difficult to heat the chips and liquor evenly by the cooking circulations. If the heating is not even some chips are not cooked as much as the others and the pulp quality is uneven and there can be a lot of uncooked material in the pulp. The bigger the temperature difference between the impregnation zone in the top and the cooking zone is the more difficult it is to reach an even heating result. Hot liquor density is lower than cold liquor density. If the density difference between the cooking zone hot liquors and impregnation zone cool liquors is too large, the hot liquor starts to channel to the top of the digester and cool liquors start to channel to the cooking zone causing severe disturbances for the cooking process. So it would be advantageous to be able to increase the impregnation zone temperature of the hydraulic digester, such as a single-vessel hydraulic digester, especially in the cases when there is only atmospheric steaming in the feed system and the temperature difference is high.
WO94/23120 describes a method in which steamed chips entrained in relatively cool liquor (at about 116° C.) are fed toward the top of a digester. The cool liquor is separated from the chips in a stand-alone separator/liquid exchanger (such as an inverted top separator) externally of the digester and replaced with hot cooking liquor (e.g. at 143° C.). The chips entrained in cooking liquor at cooking temperature are fed to the top of the digester. This process requires a free-standing liquid exchanger. Furthermore, it does not solve the problem caused by a high temperature difference in a single-vessel digester having an impregnation zone. A similar method is disclosed in U.S. Pat. No. 5,658,428, but the cool liquor is replaced with hot impregnation liquor in a liquid exchanger externally of the digester